This invention relates to imaging systems using magnetic resonance, and more specifically to methods and apparatus for determining the location of a probe within the anatomy of a patient.
Magnetic Resonance Imaging (MRI) is commonly used for diagnostic purposes. It can also be employed for other purposes, such as therapy and image-guided surgery, which is becoming more frequent. In image guided surgery, MRI is used to make an image, and the position of a surgical instrument is determined in relation to the anatomy and to the images of the patient. Other modalities have also been used, especially x-ray computer tomography (CT). For more details, see "Image-guided Access Enhances Microtherapy", in Supplement to Diagnostic Imaging November 1996.
Positioning of surgical instruments, such as catheters and biopsy needles, on the image should be made quickly and with high precision to facilitate the work of the surgeon. In some image guided surgery systems, the position of the surgical instrument is determined using information of a non-magnetic character, for example by using an optical or ultrasonic localizer. Other systems use magnetic resonance signals. The advantage of the optical method is that it is fast, allowing tracking of the instrument in real time. A disadvantage is that it needs its own optical reference frame, which has to be registered or correlated in relation to the patient and the images. The instrument also has to be in optical contact with the localizer.
An advantage of using MR signals for positioning of the surgical instrument is that spatial information may be derived from the field of the MRI gradient coils, which is also used to determine the position of the anatomical details seen in the image. This fact makes it easier to ensure that the position of the instrument is seen in the correct place in the image.
Localization using magnetic resonance methods can be passive or active. Passive methods use the normal MR image, where the instrument is made visible by attaching markers to it. Markers are formed either by choosing suitable materials for the instruments themselves, or by attaching relatively contrasty markers to the instruments such that the markers appear on the image either as a clear or a dark spot or region. Active tracking uses a small NMR coil acting on a proton sample. The signal generated thereby is amplified in a separate channel and added to the image. In this way it can be made stronger to obtain a more clear spot.
Yet another method is to use dynamic polarization to amplify the NMR signal emanating from a reference marker attached to the region of interest, for example as disclosed in U.S. Pat. No. 5,218,964 to Sepponen. With this method the same receiver coils and processing can be used as for the image.
A drawback to the passive methods is one of visibility. The marker signal has a limited strength and is prone to different types of artefacts. In certain circumstances the position of the instrument may become lost or uncertain. The active markers have the problem of complexity, which tends to make position tracking slow and/or less reliable, and often they tend to have too large a size.
As will be appreciated, a method and apparatus which provides a more accurate, yet compact and straightforward method of determining the location of a surgical instrument with respect to the anatomy is needed.